Electro-optical board assembly for measuring the temperature of an object surface from infra-red emissions thereof, including an automatic gain control therefore

ABSTRACT

An automatic gain control technique integrates samples of an incoming analog signal a controlled amount of time so that the magnitudes of the samples lie within the desired input window of an analog-to-digital converter or other signal processing device. The values of the samples are then determined from a combination of the output of the signal processing device and their integration time. This is utilized in a system for determining the temperature of a surface of an object, without contacting the surface, by measuring the level of its infra-red radiation emission. A particular application of the system is to measure the temperature of a semiconductor wafer within a processing chamber while forming integrated circuits on it. The measuring system is configured on a single printed circuit board with an extra height metal heat sink structure to which a cooling unit is mounted. A photodetector and a circuit chip which performs the signal integrations are mounted within the cooling unit and operated at a uniform temperature. The combination of the variable time integration and cooling techniques greatly increases the signal-to-noise ratio of the measuring system. This allows surface temperature measurements to be made down to about 250° C. by measuring object emissions at slightly less than one micron in wavelength, conditions which provide signal levels which are normally too weak to measure accurately.

BACKGROUND OF THE INVENTION

This invention relates generally to electronic automatic gain controltechniques, and, more specifically, to the use of such techniques in themeasurement of a level of infra-red radiation emitted by an objectsurface whose temperature is being measured.

There are many applications where the intensity of the emission from anobject surface of infra-red radiation is detected by a photodetector andmeasured in order to determine the temperature or some other parameterof the object surface. One such application utilizes a small black-bodycavity formed on an end of an optical fiber as a temperature sensor,emissions from the cavity being transmitted to a photodetector at theother end of the optical fiber. The temperature of the environment inwhich the black-body sensor is positioned is thus measured. An exampleof such a system is disclosed in U.S. Pat. No. 4,750,139. Since theintensity level of the infra-red emissions varies considerable over atemperature range of interest, as much as an order of magnitude or more,an automatic gain control circuit is used in a front end of a measuringinstrument in order to maintain a temperature proportional analog signalat a level within a desired input range of an analog-to-digitalconverter (DAC) or other signal processing device. The automatic gaincontrol is typically formed of an analog circuit including at least oneamplifier whose gain is set through a switched resistor network.

Pyrometers have also been used for a long period of time for measuringthe temperature of a surface without having any contact with thatsurface. Infra-red emissions from the surface are remotely imaged onto aphotodetector and processed in order to determine the surfacetemperature. More recently, optical fibers or other light pipes are usedas part of the infra-red radiation transmission and imaging system.

One important current application of such non-contact temperaturemeasurement is in the field of the formation of integrated circuits.Several steps of forming an integrated circuit on a semi-conductor waferare performed within a reaction chamber. The temperature of the wafersis rapidly cycled through a predetermined temperature profile in a classof processes referred to as rapid thermal processing (RTP). Thenecessity to follow predetermined temperature profiles during the timeof processing is a result of the very small dimensions of the variousregions and components of the integrated circuits so formed. Accurate,real time measurement of the wafer's temperature is thus quite importantto the success of RTP.

Almost all semi-conductor processing is performed on silicon wafers.Silicon has the characteristic of being transparent to radiation havinga wavelength in excess of about one micron. Therefore, it is usuallydesired to measure the level of infra-red emissions from a silicon waferat a wavelength of less than one micron. Heretofore, such a techniquehas been useable to measure temperatures only down to about 500° C. Thisis because the infra-red emissions from the wafer surface, as with anysuch surface, drop to a level that is so low at low temperatures thatcurrent techniques cannot accurately measure the emissions. Thesignal-to-noise ratio becomes quite low, making accurate measurementsvery difficult.

Therefore, it is a principle object of the present invention to provideindividual techniques and a combination of techniques which allow makingsuch temperature measurements with improved accuracy.

It is a specific object of the present invention to provide individualtechniques and a combination of techniques for allowing the measurementof semi-conductor wafer temperatures by monitoring emission wavelengthsless than one micron of temperatures in wider ranges than now possibleto be made with a high degree of accuracy.

It is a more general object of the present invention to provide animproved automatic gain control technique for use in the front end ofinstruments used to make such measurements.

It is an even more general object of the present invention to provide atechnique for digitizing analog signals which can vary over a range thatis much larger than a preferred input range to an analog-to-converter orother signal processing device.

It is an additional object of the present invention to provide animproved physical arrangement of optical and electronic components fordetecting the level of radiation emissions.

SUMMARY OF THE INVENTION

A first aspect of the present invention is an improved automatic gaincontrol (AGC) technique that varies a characteristic of the integrationof an electrical signal output of a photodetector receiving suchradiation emissions in order to control the magnitude of a signalapplied to a device having a limited input signal range, such as ananalog-to-digital converter. The integrating technique preferably usesonly capacitors as components, avoiding any significant amount ofresistance in the integrating circuit in order to minimize noisegenerated by the circuit. A measure of the radiation level striking thephotodetector is a combination of the analog-to-digital output and acharacteristic of the integrator that is varied in response to themagnitude of the signal presented, integration time being the preferredcharacteristic that is controlled. In addition to the pyrometricapplication of this signal processing technique, it also hasapplications elsewhere when a need exists to measure a signal capable ofsuch a wide variation. Further, in other applications, the technique isapplicable to digitizing an analog signal, whether it has a widevariation or not.

According to another aspect of the present invention, the photodetectorand integrating electronic circuit chip are mounted together within atemperature controlled chamber which is maintained at a uniform cooltemperature, thus further reducing generation of noise thereby. Such achamber is preferably mounted on a metallic heat sink surface which isshaped to have a large heat dissipating area as well as being mounted ona standard printed circuit board without seriously impeding the spacefor normal mounting of electronic components directly on the printedcircuit board. It is also preferred that the infra-red signals becommunicated by an optical fiber directly to the photodetector.

The combining of integration as an automatic gain control and themaintaining of the photodetector and integrating electronic circuit chipat a cool, uniform temperature provide an instrument that is capable ofmeasuring temperatures down to about 250° C. at an infra-red emissionwavelength of slightly less than one micron. The measurements areperformed with a high degree of accuracy and a good signal to noiseratio. This technique thus has a significant application to themonitoring of semiconductor wafer temperatures during rapid thermalprocessing of them. Accurate temperature measurements are obtained withthese techniques over a range of about 250° C. to 1200° C.

Additional objects, advantages and features of the various and severaldifferent aspects of the present invention are best understood byreference to the following description of its preferred embodiments,which description should be taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B generally illustrate temperature measurementapplications in which the various aspects of the present invention haveparticular utility;

FIG. 2 is a curve showing the intensity of an infra-red radiationemission from an object surface as a function of that surface'stemperature;

FIG. 3 is an electronic circuit diagram of a front end portion of ameasuring instrument which utilizes an integration technique forautomatic gain control;

FIGS. 4A-4F are individual signals that exist in the circuit diagram ofFIG. 3 and which, taken together, provide a timing diagram of theoperation of the circuit of FIG. 3;

FIG. 5 is a flowchart which illustrates operation of the electronicsystem illustrated in FIGS. 3 and 4A-4F;

FIG. 6 shows another system in which the various aspects of the presentinvention can be utilized;

FIG. 7A and 7B are curves which illustrate operation of the system ofFIG. 6;

FIG. 8 shows a printed circuit board structure for implementing thefront end system of FIGS. 3-5;

FIG. 9 is a cross-sectional view of a temperature controlled chamberwhich is provided as part of the configuration of FIG. 8, taken atsection 9--9 thereof;

FIG. 10 is a cross-sectional view of the temperature controlled chamberof FIGS. 8 and 9, taken at section 10--10 of FIG. 9; and

FIG. 11 is an optical diagram which schematically illustrates thestructure of another component of the physical system arrangement ofFIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One application of the various embodiments of the present invention isshown in the schematic diagram of FIG. 1A. A length of optical fiber 11is provided at one end with a black-body cavity 13 attached to thefiber. Example black-body structures of this type are disclosed in U.S.Pat. No. 4,576,486 and allowed U.S. application Ser. No. 07/874,829, thedisclosure of which is incorporated herein by this reference. Theblack-body cavity 13 is positioned within an environment 15 whosetemperature is to be measured, usually an enclosed space being heated bya heat source 17. The black-body cavity 13 is thus heated to thetemperature of the environment 15 in which it is placed. Infra-redradiation emissions of the black-body 13 are communicated out of theenvironment 15 though the optical fiber 11. The spectrum and relativeintensities of these emissions as a function of the temperature of theblack body 13 can be ascertained from well known principles.

This temperature dependent infra-red radiation emission is communicatedby the optical fiber 11 to a measuring instrument 19. It is thisinstrument which includes embodiments of the present invention describedbelow. Briefly, a photodetector converts the infra-red emissionsreceived from the black-body cavity 13. An electrical signal output ofthe photodetector is electronically processed in order to output signalswhich indicate the temperature of the black-body 13. One output 21carries that temperature measurement signal in digital form to a hostcomputer system 23 or other utilization device. Such a host computersystem includes a usual output device 25 for communicating thetemperature measurement, and an input device such as a keyboard 27. Asecond output 29 of the instrument 19 provides an analog signalproportional to the temperature of the black-body cavity 13. Either thedigital output 21 or analog output 29 can, in addition to simplyproviding an indication of the temperature within the environment 15, beused to control the heat source 17 or some other parameter of a processin response to that temperature.

Another application of black-body temperature measurement techniques isillustrated in FIG. 1B. In this case, rather than having a black-bodycavity attached to an end of the optical fiber, a length of opticalfiber 11' receives infra-red emissions directly from a surface of anobject 31 whose temperature is to be determined. The bare end of theoptical fiber 11' can serve to gather radiation emitted from the object31, or can be supplemented by the use of lenses or other opticalelements (not shown) to gather a larger field of radiation emissionsinto the end of the optical fiber 11'. An example application of thetechnique of FIG. 1B is where the object 31 is a semi-conductor waferbeing processed within a processing chamber 33. As part of that process,the temperature of the wafer 31 is carefully controlled by some heatsource 35. The ability to be able to monitor the temperature of thewafer itself is becoming increasingly important with advancedsemi-conductor processing technology. The type of heat source 31utilized in such processes is selected from heating lamps, radiofrequency sources, and the like.

Referring to FIG. 2, an example characteristic curve is given whichrelates the intensity of a defined bandwidth of infra-red radiationemissions (x-axis) with the temperature (y-axis) of the object whoseemissions are being measured. Various commercial pyrometers arecalibrated to a specific curve of the type of FIG. 2 which is derivedfrom well known Plank's black-body emission characteristics. Such acurve, when used in a measuring instrument such as the instrument 19, iscalibrated to take into account the emissivity of the surface that isemitting the infra-red radiation being detected.

It will be noted from the example curve of FIG. 2 that the radiationlevel (x-axis) is a logarithmic scale. A measuring instrument must thusbe able to handle an incoming optical signal having an intensity thatvaries over an extremely wide range, often by an order of magnitude (tentimes) or more.

A typical pyrometric temperature measuring instrument digitizes thedetected optical signal in order to measure where on the curve of FIG. 2the emission level exists. Typically, an analog-to-digital converterused in such instruments has a linear input range that is much smallerthan that of the detected optical signal over the full temperature rangeof the measuring instrument. Thus, some form of automatic gain controlcircuitry is utilized to keep the incoming signal within this operatingwindow of the analog-to-digital converter. This is usually accomplishedby the use of amplifiers having switched resistor networks to adjust theamplifier gain. Such use of resistors inherently generates electricalnoise. This and noise generated by the photodetector itself has limitedhow low a temperature can be measured by such an instrument. It is atthe lower temperatures that the optical signal is low, and thus theelectrical signal output of the photodetector which is being processedis low with respect to noise being generated in the circuit.

Therefore, the present invention includes a significant departure fromthe usual automatic gain control technique, an implementation thereofbeing shown in the schematic diagram of FIG. 3. The example shown inFIG. 3 has two signal channels. A photodetector 41 receives an opticalsignal from an optical fiber 43, and a photodetector 45 receives anoptical signal from an optical fiber 46. Output signals of thephotodetectors in respective lines 47 and 49 are applied to acommercially available integrated circuit chip 51 which contains twoidentical integrating circuits. The structure and operation of only onechannel and integrating circuit is described herein, it being recognizedthat the other channel is the same.

One channel of the circuit chip 51 includes a differential amplifier 53having a feedback capacitor C1 connected between its output and aninverting input. A non-inverting input is connected to ground potential.An electrical signal output of the photodetector 41 is connected througha hold switch 55 to the inverting input of the amplifier 53. Its outputis connected though a select switch 57 to form an output in a line 59 ofthe chip 51. A reset switch 61 is connected across the feedbackcapacitor C1. Each of the switches 55, 57 and 61 is operated betweenconductive and non-conductive states in response to respective controlsignals in lines 63, 65 and 67 from outside of the circuit chip 51, suchas from a controller 69. The circuit chip 51 is preferably part numberACF2101 from the Burr-Brown Corporation.

The integrator operates, when its hold switch 55 is closed and its resetswitch 61 and select switch 57 are opened, to accumulate a charge in itscapacitor C1 having a magnitude dependent upon the level of currentflowing in line 47 from the photodetector 41 and the amount of time thatthe hold switch 55 is closed. The current flowing in the line 47 isproportional to the level of the optical signal striking thephotodetector 41 through the optical fiber 43. After that step, thecharge on the capacitor C1 is transferred to another capacitor C2 thatis outside of the integrator chip 51 by closing the select switch 57while the hold switch 55 and the reset switch 61 are opened. The chargein the capacitor C2 is then amplified by a buffer amplifier 71, whoseoutput becomes an input to an analog-to-digital converter 73. A digitaloutput in lines 75 is connected back to the controller 69. The digitalsignal in line 75 gives a measure of the intensity of the infra-redradiation striking the photodetector 41 during the initial integrationcycle. Once such a sample is acquired, the reset switch 61 is closed todischarge the capacitor C1 and another integrating cycle begun.

The temperature of the object emitting the radiation which was detectedis determined by the controller from the digital signal on the line 75.The controller includes an integral of Plank's black-body curve whichhas been corrected for the transfer characteristics of the photodiode41. This is a curve similar to that shown in FIG. 2. Alternatively, thethe information of such a curve can be contained in a look-up tablewithin memory of the controller 69. In either case, that informationallows the controller to convert the infra-red radiation intensity givenin lines 71, when combined with the time that the hold switch 55 washeld closed by a control signal in the line 63 to acquire the sample, inorder to ascertain the emitting object's temperature. The controller 69closes the hold switch 55 for a length of time during the acquisition ofeach sample which results in the measurement signal in line 75indicating that an input to the analog-to-digital converter 73 is withina desired window of operation wherein the ADC is most linear. If the ADCoutput signal in line 75 indicates that its input is beyond that desiredwindow, then the controller 69 adjusts the time that the hold switch 55is maintained closed when acquiring the next sample. Although thecontroller 69 can calculate temperature each time a new sample of theincoming signal is acquired, it is preferable to first average thevalues of many samples and then apply them against the black-body curveor look-up table.

The variation of integration time in response to the measurementsoperates provides the automatic gain control (AGC) function. Integrationtime, however, is not the only integration characteristic which can becontrolled. Alternatively, the integration time could be held fixedwhile the value of the feedback capacitor C1 is varied. A combination ofvarying integration time and the size of the capacitor C1 could even beimplemented. However, the ability to vary the capacitor C1 requires aswitched capacitor network which consumes a great deal of space on thecircuit chip, and, for that reason, is not preferred.

In order to compensate for gain variations in the buffer amplifier 71which will occur over time and under different operating conditions,it's non-inverting input is periodically connected through a switch 77to a reference voltage source V_(REF1) while the select switch 57 isheld open. The voltage V_(REF1) is that which the analog-to-digitalconverter 73 requires for such a reference adjustment. Once themeasurement at the output of the buffer amplifier 71 has been made whenconnected to this reference voltage source, future readings are thenadjusted by the analog-to-digital converter 73 to compensate for anyerror that exists. Another compensation is performed, as describedbelow, to correct for any offsets introduced into the signal by theintegration circuit, and perhaps also by the photodetector. Each ofthese reference and offset measurements is periodically made in order toascertain correction factors which are thereafter applied to the dataacquired until the next such measurements are made.

It will be noted that the signal path of FIG. 3 from the photodetectorto the digital-to-analog 73 contains no resistors. This eliminates asource of noise which becomes significant when dealing with low-levelsignals. Rather than adjusting a level of amplification in suchfront-end circuits by switching resistors and feedback groups ofamplifiers, as is normally done to adjust the signal level within adesired window of the analog-to-digital converter 73, the integrationtime is controlled instead. That is, the length of time that the holdswitch 55 remains closed during the acquisition of one sample controlsthe level of voltage that the output of the buffer amplifier 71 to bewithin the desired range. Physically, in order to further reduce anynoise generation within these front-end circuits, the integration chip51 and the photodetectors 41 and 45 are contained within a temperaturecontrolled compartment 78. A temperature control system includes acooler 79 and a thermistor 81 within the compartment 78. A current in aline 83 from a constant current source (not shown) is passed though thethermistor 81. A voltage drop across the thermistor 81 is compared bydifferential amplifier 85 with that set by the user through apotentiometer 87. The potentiometer sets the temperature that is desiredto be maintained within the compartment 78. An output of the amplifier85 controls a variable current source 89 which supplies the cooler 79. Aphysical arrangement for that shown schematically in FIG. 3 is describedbelow with respect to FIGS. 8-10.

Operation of the integrating circuit described with respect to FIG. 3 toperform an automatic gain control (AGC) function will be described withrespect to FIGS. 4 and 5. A step 91 of the processing flowchart of FIG.5 calls for making the periodic offset and reference measurements. Theoffset measurement occurs during interval 93 (FIG. 4). During thisinterval, the select switches 57 (FIG. 4C) and reset switch 61 (FIG. 4B)are closed while the input hold switch 55 (FIG. 4A) is opened. Thus, anysignal generated by the integrating amplifier 53 when no signal isapplied is measured, that offset then being used to correct datasubsequently acquired.

Similarly, as part of the step 91 (FIG. 5), a next interval of time 95(FIG. 4) is when the gain of the buffer amplifier 71 (FIG. 3) isreferenced. During this interval, the reference switch 77 is closed(FIG. 4D) the only time during which switch 77 is in that state. Thereference voltage V_(REF1) is connected to the input of theanalog-to-digital converter 73. That voltage is, for a typical ADC part,at a positive end of its input window, even though signals arepreferably received in a desired range 97 (FIG. 4F) which is a muchsmaller range than the full input window. After the interval 95, thenecessary corrective factors have been determined for a next subsequentnumber N of temperature measuring samples to be taken before the offsetand amplifier gain reference steps are performed again.

A next step 97 (FIG. 5) then acquires one sample of the magnitude of theincoming optical signal an example of one such cycle occurs during aninterval 99, and another example of such a cycle with a different inputsignal level occurs during an interval 101 (FIG. 4F). In each suchcycle, there is a reset period, wherein the reset switch 61 is closed todischarge the capacitor C1 (FIG. 3). That is followed by an integrationperiod, wherein the reset switch 61 is opened and the hold switch 55 isclosed to acquire an incoming signal. The duration of the integrationinterval is controlled by the controller 69 to keep the magnitude of thesignal transferred to the capacitor C2, and thus that applied to theinput of the analog-to-digital converter 73, within a predeterminedrange. At the end of the integration portion of each of the intervals 99and 101, the charge accumulated in the capacitor C1 is transferred in areadout phase to the capacitor C2 by closing the select switch 57 andmaintaining the reset switch 61 off. The hold switch 55 is also turnedoff during this final phase of each of the time intervals 99 and 101.

It will be noted from FIG. 4E, that the example being illustrated has ahigher input signal level during the interval 99 than during theinterval 101. Thus, an integration time 103 (FIG. 4E) during theinterval 99 is less than the integration time 105 during the interval101. It is the control of the integration time by the controller 69(FIG. 3) that provides the automatic gain control. This can be seen fromFIG. 4F, the voltage applied to the input of the analog-to-digitalconverter 73 during each of the intervals 99 and 101 remaining withinthe desired range 97, even though the input signal is significantlydifferent.

Returning to FIG. 5, a next step 107 results in the output of theanalog-to-digital converter 73 in line 75 to be read by the controller69. A next step 109 compares that output with an output corresponding tothe desired input range 97, in order to determine whether it is outsideof that range or not. If so, a next step 111 causes the integration time(103 and 105 in the example of FIG. 4) to increase or decreaseaccordingly to adjust the signal level applied to the input of theanalog-to-digital converter 73. Once that adjustment is made, theprocess cycles back to step 97 to acquire another sample with the newintegration time.

On the other hand, if the measurement is within the desired range of theADC 73, a next step 113 adjusts the measurement received by the lastoffset measurement. In a step 115, it is the adjusted output of theanalog-to-digital converter 73 that is applied by the controller 69 toits internal temperature table for equation. Also applied as an input tosuch a look-up table or equation is the amount of integration time (103or 105 in the example of FIG. 4) by which the sample was acquired. Anoutput of that table, indicated in a step 117, then provides atemperature measurement which is made available through outputs 21 and29 of the controller 69.

As a further step 119 in the processing, it is determined whether acertain number N measuring cycles have occurred since the last time thatthe offset measurement was made and the amplifier gain referenced. Ifso, the processing returns by a path 121 to repeat the offset and gainreference step 91. If not, the processing returns by a path 123 to step97 to conduct another measurement cycle as described with respect to theintervals 99 and 101 of FIG. 4.

FIG. 6 shows the use of the dual channel instrument illustrated in FIG.3 for a somewhat different use. A semi-conductor wafer 31' within aprocessing chamber 33' is heated, in this illustrative example, byheating lamps 33'. One of the optical fiber inputs 11' to the measuringinstrument 19 receives infra-red emissions from the wafer 31', much asin the system illustrated in FIG. 1B. However, in the application ofFIG. 6, the signal captured by the optical fiber 11' also includesradiation reflected by the wafer 31' from the lamps 33'. In order toenable the instrument 19 to separate the temperature dependent infra-redemissions from the reflected radiation, the second input optical fiber12 acquires a signal from the heating lamps themselves, or from a dummylamp of the same type which is heating the wafer 31'.

Since the wafer heating lamps 33' are driven by an alternating current,the output intensity has a small ripple that is illustrated in FIG. 7A.The signal of FIG. 7A is acquired by the optical fiber 12. A combinedsignal illustrated in FIG. 7B is that acquired by the optical fiber 11'.These two signals are initially digitized by the circuit of FIG. 3, thedigital samples then being processed to obtain a measure of thetemperature or other characteristic of the surface of the wafer 31'.This process is described in U.S. Pat. No. 5,310,260 of Schietinger etal., which is incorporated herein by this reference.

A physical arrangement for a main circuit board within the instrument 19is illustrated in FIG. 8. A usual printed circuit board 125 has a metalshell 127 mounted on it. The shell 127 is generally opened at its bottomsides and ends, except that an end 129 is enclosed. The metal shell 127is sized to have a thickness and a maximum dimension from the printedcircuit board 125 (overall thickness) so that the entire assembly can beinserted into a usual computer system and take the space of no more thanthree printed circuit board slots.

In addition to providing support for optical assemblies 131 and 133attached to the top surface of the shell 127, its primary surface is theserve as a heat sink for an enclosure 135 for the integrating circuitchip and photodetectors. A sub-board 137 is carried by the main board125 in an orientation substantially perpendicular thereto and a shortdistance away from the end surface 129 of the shell 127. The sub-board137 carries the capacitor C2, buffer amplifier 71, digital to analogconverters 73, reference switch 77, and associated components, allindicated by 138, and interconnections therewith. Similar components forthe second channel are also included on the sub-board 137. The sub-boardis positioned to be close to and on an opposite side of the shell endwall 129 from the container 137 which forms the chamber 77 in which theintegrating circuit chip and photodiodes are positioned. Chips 139forming the controller 69 are mounted on the main board 125 under theshell 127. Connectors 145 and 147 on opposite ends of the circuit board125 are selected appropriately for interconnection with whatever systemthe board of unit of FIG. 8 is to be used.

Referring to FIGS. 9 and 10, the structure of the cooling unit 135 isdescribed. Within its temperature control chamber 77 is the electricallydriven cooler 79 which is mounted on a metal heat sink 151. The heatsink 151 is firmly attached to and contacts the metal end 129 of theheat sink shell 127. Thus, heat removed from the compartment 78 by thecooling units 79 is dissipated in the atmosphere by conduction throughthe heat sink 151 and the wall 129 of the shell 127.

Each of the optical elements 131 and 133 contains an optical filter 155,as shown in FIG. 11. A pair of lenses 157 and 159, or other convenientoptical system, images the infra-red radiation signal from an end of theexternal optical fiber 11 into an end of the internal optical fiber 43.The purpose of the filter 155 is to limit the bandwidth of the radiationwhich is allowed to pass onto the associated photodetector. In a casewhere the temperature of a silicon wafer is being monitored, thatwavelength band is preferably a very narrow one with a high wavelengthand adjacent one micron.

Optionally, each of the optical units 131 and 133 may also include ashutter 161 that is moveable by a solenoid 163, or other convenientmotive force, to a position that closes off an end of the optical fiber43. If all the radiation is blocked from entering the optical fiber 43,the offset measurement made during the interval 93 (FIG. 4) is thenmodified to be made with the hold switch 55 in a closed position (on).Thus, the photodetector 41 is included in the offset measurement. Theshutter 161 would at all other times be removed from the end of theoptical fiber 43. The shutter 161 is so operated by a signal in a line165 from the controller 69.

It will be noted from FIGS. 8 and 9 that the internal optical fibers 43and 46 communicate between the optical units 131, 133 and respectivephotodetectors 45, 41. The ends of these fiber links are abutteddirectly against their respective photodetectors. This arrangementprevents the introduction of unwanted optical signals into the systemand avoids cross-talk between the two channels.

Although the various aspects of the present invention have beendescribed with respect to its preferred embodiments, it will beunderstood that the invention is entitled to protection within the scopeof the appended claims.

It is claimed:
 1. A method of measuring the temperature of an objectfrom a level of intensity of infra-red radiation emitted by the object,comprising:positioning a photodetector to receive said infra-redradiation emission, thereby to generate an electrical signalproportional thereto, periodically integrating said electrical signal, acharacteristic of the individual integrations being controllable,applying the integrated signal to an analog-to-digital converter havingan optimum input signal range, controlling said characteristic of theindividual integrations in order to maintain the integrated signalwithin said optimum input signal range, and determining the temperatureof the object from both a value of said characteristic and an output ofsaid analog-to-digital converter.
 2. A method according to claim 1,additionally comprising periodically measuring the output of saidanalog-to-digital converter without any temperature proportional signalapplied thereto, thereby to measure an offset value, and further whereinsaid offset value is utilized when determining the temperature of theobject.
 3. A method according to claim 2 wherein any temperatureproportional signal is blocked from application to the input of theanalog-to-digital converter by opening an electrical signal path betweenthe photodetector and the analog-to-digital converter input.
 4. A methodaccording to claim 2 wherein any temperature proportional signal isblocked from application to the input of the analog-to-digital converterby blocking the photodetector from receiving said infra-red radiationemission.
 5. A method according to claim 1, additionally comprisingmaintaining the photodetector and performing the signal integrationwithin a cooled temperature controlled enclosure.
 6. A method accordingto claim 1 wherein the controlled characteristic of the individualintegrations is the time of the individual integrations.
 7. A methodaccording to claim 1 wherein the photodetector positioning includespositioning an optical fiber signal path between the object and thephotodetector, including physically abutting one end of the opticalfiber signal path against the photodetector.
 8. A method according toclaim 7 wherein the infra-red radiation emitted by the object isgathered into another end of the optical fiber signal path withoutphysical contact therewith.
 9. A method according to claim 7 wherein theobject is attached to another end of the optical fiber signal path. 10.A method according to any one of claims 1-8 wherein said object includesa semiconductor wafer positioned within an integrated circuit processingchamber.
 11. A method according to any one of claims 2-5 or 7-9 whereinthe controlled characteristic of the individual integrations is the timeof the individual integrations.
 12. A method of measuring acharacteristic of a surface of an object being heated by lamps whichemit infra-red radiation with a time varying ripple component,comprising:detecting a combination of radiation reflected by a surfaceof the object from the lamps and radiation emitted by said surface in amanner to obtain a first electrical signal proportional thereto,directly detecting the lamp radiation in a manner to obtain a secondelectrical signal proportional thereto, digitizing each of the first andsecond electrical signals by a method which comprises:periodicallyintegrating said electrical signal, the time of the individualintegrations being controllable, applying the integrated signal to ananalog-to-digital converter having an optimum input signal range,controlling the time of the individual integrations in order to maintainthe integrated signal within said optimum input signal range, andobtaining digital values of said electrical signal from both the time ofthe individual integrations and an output of said analog-to-digitalconverter, determining a magnitude of a time varying component in eachof said first and second electrical signals after digitation thereof,and combining at least the time varying components of said first andsecond digitized electrical signals in a manner to determine said objectsurface characteristic.
 13. A method according to claim 12 wherein thesurface characteristic being measured is its temperature.
 14. A methodaccording to claim 13, wherein the digitizing of each of said first andsecond electrical signals additionally comprises periodically measuringthe output of said analog-to-digital converter without any signalproportional to detected radiation being applied thereto, thereby tomeasure an offset value, and further wherein said offset value isutilized when obtaining digital values of the electrical signal.
 15. Amethod according to claim 12, wherein the digitizing of each of saidfirst and second electrical signals additionally comprises performingthe integration of each signal on a single integrated circuit chip thatis held within a cooled temperature controlled enclosure.
 16. A methodaccording to any one of claims 12-15 wherein said object is asemiconductor wafer having integrated circuits being formed thereon. 17.A method of digitizing an analog signal output of a photodetector thatcorresponds to an optical signal being received by said photodetectorand having a characteristic related to a parameter being measured byapplying said analog signal to an analog-to-digital converter which ischaracterized by having an optimum input signal range,comprising:acquiring individual samples of the analog signal byrepetitively integrating said analog signal for times that place thesamples within said optimum signal range, determining the values of theanalog signal samples from an output of said analog-to-digital converterand the time of integration, and relating the determined values of theanalog signal samples to said parameter.
 18. The method according toclaim 17, wherein the parameter being measured includes a characteristicof a surface of an object that emits said optical signal.
 19. The methodaccording to claim 18, wherein said characteristic includes thetemperature of the object surface.
 20. The method according to claim 19,wherein said object includes a semiconductor wafer.
 21. In an electronicsystem receiving an analog signal from an electrical output of aphotodetector and applying it to an input of a signal processing devicewhich is characterized by having a defined input signal range windowthat is smaller than swings in the incoming analog signal, an automaticgain control circuit, comprising:means receiving said incoming analogsignal for integrating individual samples thereof for a controllabletime, means controlling the time that the individual signal samples areintegrated for maintaining said signal integrated samples within saiddefined input signal range window, and means responsive to both the timethat the individual signal samples are integrated and an output of saidsignal processing device for determining the values of the individualsignal samples.
 22. The system of claim 21 wherein the signal processingdevice includes an analog-to-digital converter.
 23. The system of claim21 which additionally comprises a length of optical fiber positioned todirect an optical signal onto said photodetector.